Electrolyte for lithium battery and lithium battery comprising same

ABSTRACT

The electrolyte for a lithium battery includes a non-aqueous organic solvent, a lithium salt, and an additive of the following Formula 1: 
     
       
         
         
             
             
         
       
         
         
           
             wherein, X and Y are the same or different from each other and selected from O, S, CR 2 , or NR, where here R is H, a halogen, or an alkyl having a carbon number of less than or equal to 8, or neighboring alkyl groups are combined to each other to form a cycle or hetero cycle, and both X and Y are not oxygen (O),
 
R a  to R d  are the same or different from each other and selected from H, a halogen, an alkoxy group having a carbon number of less than or equal to 8, or an unsaturated or saturated alkyl group having a carbon number of less than or equal to 8, or neighboring alkyl groups are combined to each other to form a cycle or hetero cycle, and at least one of R a  to R d  is a halogen.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean patentapplication No. 10-2004-0088689 filed in the Korean IntellectualProperty Office on Nov. 3, 2004, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an electrolyte for a lithium battery,and a lithium battery comprising the same, and more particularly, to anelectrolyte for a lithium battery which inhibits overcharging of thebattery, and a lithium battery comprising the same.

BACKGROUND OF THE INVENTION

The use of portable electronic instruments is increasing as electronicequipment gets smaller and lighter due to developments in high-techelectronic industries.

Studies on lithium secondary batteries are actively being pursued inaccordance with the increased need for a battery having high energydensity for use as a power source in these portable electronicinstruments. Such a lithium secondary battery, having an averagedischarge potential of 3.7V (i.e., a battery having substantially a 4Vaverage discharge potential) is considered to be an essential element inthe digital generation since it is an indispensable energy source forportable digital devices such as cellular telephones, notebookcomputers, camcorders, etc. (i.e., the “3C” devices).

Also, there has been extensive research on batteries with effectivesafety characteristics such as preventing overcharging.

When a battery is overcharged, an excess of lithium ions is deposited ona positive electrode, and an excess of lithium ions is also insertedinto a negative electrode, making the positive and negative electrodesthermally unstable. An eruptive explosion occurs from a decomposition ofthe electrolytic organic solvent, and the thermal runaway that occurscauses serious problems of battery safety.

To overcome the above problems, it has been suggested that an aromaticcompound such as an oxidation-reduction additive agent (“redox shuttle”)be added to the electrolyte. For example, U.S. Pat. No. 5,709,968discloses a non-aqueous lithium ion secondary battery to prevent thermalrunaway resulting from an overcharge current by using a benzene compoundsuch as 2,4-difluoroanisole. U.S. Pat. No. 5,879,834 discloses a methodfor improving battery safety by using a small amount of an aromaticcompound, such as biphenyl, 3-chlorothiophene, furan, etc., which iselectrochemically polymerized to increase the internal resistance of abattery during unusual overvoltage conditions. Such redox shuttleadditives increase the temperature inside the battery early due to heatproduced by the oxidation-reduction reaction, and close pores of aseparator through quick and uniform fusion of the separator to inhibitan overcharge reaction. The polymerization reaction of these redoxshuttle additives consumes the overcharge current to improve batterysafety.

However, the need for high capacity batteries is increasing, and theseredox shuttle additives cannot provide the high level of safety requiredof such high capacity batteries. Therefore, a need exists for anelectrolyte capable of preventing overcharge and ensuring batterysafety.

SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention provides an electrolytefor a lithium battery capable of improving battery safety.

Another embodiment of the present invention provides a lithium batterywhich includes the above electrolyte.

According to one embodiment of the present invention, an electrolyte fora lithium battery includes a non-aqueous organic solvent, a lithiumsalt, and an additive. The additive is represented by the followingFormula 1:

wherein, X and Y are the same or different from each other and selectedfrom O, S, CR₂, or NR, where here R is H, a halogen, or an alkyl havinga carbon number of less than or equal to 8, or neighboring alkyl groupsare combined to each other to form a cycle or hetero cycle, and both Xand Y are not oxygen (O),

R_(a) to R_(d) are the same or different from each other and selectedfrom H, a halogen, an alkoxy group having a carbon number of less thanor equal to 8, or an unsaturated or saturated alkyl group having acarbon number of less than or equal to 8, or neighboring alkyl groupsare combined to each other to form a cycle or hetero cycle, and

at least one of R_(a) to R_(d) is a halogen.

According to another embodiment of the present invention, a lithiumbattery is provided, which includes the above electrolyte, a positiveelectrode including a positive active material being capable ofintercalating and deintercalating lithium ions, and a negative electrodeincluding an active material selected from the group consisting of amaterial being capable of intercalating/deintercalating lithium ions, alithium metal, a lithium-containing alloy, and a material being capableof forming a lithium-containing compound by reversibly reacting lithium.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing, which is incorporated in and constitutes apart of the specification, illustrates embodiments of the invention, andtogether with the description, serves to explain the principles of theinvention.

FIG. 1 is a cross-sectional view of a non-aqueous lithium battery cell.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, embodiments of the invention havebeen shown and described, simply by way of illustration of the best modecontemplated by the inventors of carrying out the invention. As will berealized, the invention is capable of modification in various respects,all without departing from the invention. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and notrestrictive.

A cross-sectional view of a general non-aqueous Li-ion cell is shown inFIG. 1.

The Li-ion cell 1 is fabricated by inserting an electrode assembly 8including a positive electrode 2, a negative electrode 4, and aseparator 6 interposed between the positive and negative electrodes 2and 4 into a battery case 10. An electrolyte 26 is injected into thebattery case 10 and impregnated into the separator 6. The upper part ofthe case 10 is sealed with a cap plate 12 and a sealing gasket 14. Thecap plate 12 has a safety vent 16 to release pressure. A positiveelectrode tab 18 and a negative electrode tab 20 are respectivelyattached on the positive electrode 2 and the negative electrode 4.Insulators 22 and 24 are installed on the side and lower parts of theelectrode assembly 8 to prevent a short circuit occurrence in thebattery.

In a lithium battery, the temperature of the battery increases abruptlybecause of overcharging due to incorrect operation or break-down of thebattery, or a short circuit occurrence due to a defect in the batterydesign, so that thermal runaway takes place. During overcharging, anexcessive amount of lithium ions are released from the positiveelectrode and deposited on the surface of the negative electrode torender the positive and negative electrodes unstable. As a result,exothermic reactions such as pyrolysis of an electrolyte, reactionsbetween the electrolyte and lithium, an oxidation reaction of theelectrolyte on the positive electrode, a reaction between theelectrolyte and oxygen gas that is generated from the pyrolysis of thepositive active material, etc., rapidly increase the temperature insidethe battery to cause thermal runaway, and thus, the generation of fireand smoke.

In order to address the above problems, in the present invention acompound represented by the following Formula 1 is used as anelectrolyte additive to improve battery safety on overcharging:

wherein, X and Y are the same or different from each other and selectedfrom O, S, CR₂, or NR, where here R is H, a halogen, or an alkyl havinga carbon number of less than or equal to 8, or neighboring alkyl groupsare combined to each other to form a cycle or hetero cycle, and both Xand Y are not oxygen (O),

R_(a) to R_(d) are the same or different from each other and selectedfrom H, a halogen, an alkoxy group having a carbon number of less thanor equal to 8, or an unsaturated or saturated alkyl group having acarbon number of less than or equal to 8, or neighboring alkyl groupsare combined to each other to form a cycle or hetero cycle, and

-   -   at least one of R_(a) to R_(d) is a halogen.

One preferred example of the additive compound is represented by thefollowing Formula 2:

-   -   wherein, R¹ and R² are the same as R in the definition of Y₁,        and R³ to R⁶ are the same as R_(a) to R_(d) in the above        Formula 1. That is to say, R³ to R⁶ are the same or different        from each other and selected from H, a halogen, an alkoxy group        having a carbon number of less than or equal to 8, or an        unsaturated or saturated alkyl group having a carbon number of        less than or equal to 8, or neighboring alkyl groups are        combined to each other to form a cycle or hetero cycle, and at        least one of R³ to R⁶ is a halogen.

Another preferred example of the additive compound is a compoundrepresented by the following Formula 3 where in the above Formula 1, Xis O, and Y is CH₂:

-   -   wherein, R_(a) to R_(d) are the same or different from each        other and selected from H, a halogen, an alkoxy group having a        carbon number of less than or equal to 8, or an unsaturated or        saturated alkyl group having a carbon number of less than or        equal to 8, or neighboring alkyl groups are combined to each        other to form a cycle or hetero cycle, and at least one of R_(a)        to R_(d) is a halogen.

Another exemplary compound is represented by the Formula 4:

-   -   wherein, R¹ and R² are the same as R in the definition of X, R³        and R⁴ are the same as R in the definition of Y, and R⁵ to R⁸        are the same as R_(a) to R_(d) in the above Formula 1. That is        to say, R⁵ to R⁸ are the same or different from each other and        selected from H, a halogen, an alkoxy group having a carbon        number of less than or equal to 8, or an unsaturated or        saturated alkyl group having a carbon number of less than or        equal to 8, or neighboring alkyl groups are combined to each        other to form a cycle or hetero cycle, and at least one of R⁵ to        R⁸ is a halogen.

Another exemplary compound is represented by the Formula 5:

-   -   wherein, R_(a) to R_(d) are the same or different from each        other and selected from H, a halogen, an alkoxy group having a        carbon number of less than or equal to 8, or an unsaturated or        saturated alkyl group having a carbon number of less than or        equal to 8, or neighboring alkyl groups are combined to each        other to form a cycle or hetero cycle, and at least one of R_(a)        to R_(d) is a halogen.

Another exemplary compound is represented by the Formulas 6 to 15.

The additive compound represented by the above Formula 1 initiatespolymerization at more than about 4.5V to coat an electrode surface andincrease resistance between positive and negative electrodes.Alternatively, the compound performs an oxidation/reduction reaction ata voltage of more than about 4.5V to consume an applied current atovercharge and ensure safety of a lithium battery.

The additive compound of the above Formula 1 may be used in an amount of0.01 to 50 wt %, preferably 0.01 to 30 wt %, more preferably 0.01 to 10wt %, and still more preferably 0.01 to 5 wt %, based on the totalweight of the electrolyte. When the amount of the compound is less than0.1 wt %, the electrolyte may be ignited. When it is more than 50 wt %,a battery performance may be deteriorated.

The additive compound is added to a non-aqueous organic solventincluding a lithium salt. The lithium salt acts as a supply source oflithium ions in the battery, making the basic operation of a lithiumbattery possible. The non-aqueous organic solvent plays a role of amedium wherein ions capable of participating in the electrochemicalreaction are mobilized.

The lithium salt is preferably at least one selected from the groupconsisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃,Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x and y are naturalnumbers), LiCl, LiI, and a mixture thereof.

The concentration of the lithium salt preferably ranges from 0.6 to2.0M, more preferably 0.7 to 1.6M. When the concentration of the lithiumsalt is less than 0.6M, the electrolyte performance deteriorates due toits ionic conductivity and when the concentration of the lithium salt isgreater than 2.0M, the lithium ion mobility decreases due to an increaseof the electrolyte viscosity.

The non-aqueous organic solvent may include a carbonate, an ester, anether, or a ketone. Examples of carbonates include dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylenecarbonate (BC). Examples of esters include n-methyl acetate, n-ethylacetate, n-propyl acetate, etc.

It is preferable to use a mixture of a chain carbonate and a cycliccarbonate. The cyclic carbonate and the chain carbonate are preferablymixed together in a volume ratio of 1:1 to 1:9. When the cycliccarbonate and the chain carbonate are mixed in the volume ratio of 1:1to 1:9, and the mixture is used as an electrolyte, electrolyteperformance may be enhanced.

In addition, the electrolyte of the present invention may furtherinclude mixtures of the carbonate solvents and aromatic hydrocarbonorganic solvents. For the aromatic hydrocarbon organic solvent, asolvent represented by the following Formula 16 may be used. Examples ofaromatic hydrocarbon solvents include benzene, fluorobenzene, toluene,trifluorotoluene, chlorobenzene, and xylene.

-   -   wherein, R¹¹ is a halogen or a C₁ to C₁₀ alkyl, and n is an        integer of 0 to 6.

The carbonate solvents and the aromatic hydrocarbon solvents arepreferably mixed together in a volume ratio of 1:1 to 30:1. When acarbonate solvent and an aromatic hydrocarbon solvent are mixed witheach other in the aforementioned volume ratio, and the mixture is usedas an electrolyte, electrolyte performance may be enhanced.

The present invention provides a lithium battery including the aboveelectrolyte. For a positive active material, a compound being capable ofintercalating/deintercalating lithium (a lithiated intercalationcompound) reversibly is used. For the negative active material, acarbonaceous material being capable of intercalating/deintercalatinglithium, a lithium metal, a lithium-containing alloy, and a carbonaceousmaterial being capable of reversibly forming a lithium-containingcompound by reacting lithium is used.

A lithium battery is prepared by the following process. A negativeelectrode and a positive electrode are fabricated by a conventionalprocess, an insulating resin with a network structure is interposedbetween the negative and positive electrodes, and then the whole iswound or stacked to fabricate an electrode assembly. Then, the electrodeassembly is inserted into a battery case, followed by sealing. Theseparator is a polyethylene or polypropylene monolayered separator, apolyethylene/polypropylene double layered separator, apolyethylene/polypropylene/polyethylene three layered separator, or apolypropylene/polyethylene/polypropylene three layered separator. Across-sectional structure of the lithium battery prepared by the aboveprocess is shown in FIG. 1.

The lithium battery includes a non-rechargeable lithium battery or arechargeable lithium battery.

The lithium battery including the electrolyte of the present inventionhas improved overcharge inhibiting properties over a battery including aconventional non-aqueous electrolyte.

The following examples further illustrate the present invention indetail, but are not to be construed to limit the scope thereof.

COMPARATIVE EXAMPLE 1

94 g of LiCoO₂ as a positive active material, 3 g of Super P (acetyleneblack) as a conductive agent, and 3 g of polyvinylidenefluoride (PVdF)as a binder were mixed in N-methyl-2-pyrrolidone (NMP) to prepare apositive slurry. The slurry was coated on an aluminum foil having awidth of 4.9 cm and a thickness of 147 μm, dried, compressed, and thencut into a predetermined size, thus manufacturing a positive electrode.

90 g of mesocarbon fiber (MCF from PETOCA Company) as a negative activematerial and 10 g of PVdF as a binder were mixed to prepare a negativeslurry. The slurry was coated on a copper foil having a width of 5.1 cmand a thickness of 178 μm, dried, and compressed, and then cut into apredetermined size, thus manufacturing a negative electrode.

Between the manufactured positive and negative electrodes, apolyethylene film separator was interposed followed by winding the wholeto fabricate an electrode assembly. The manufactured electrode assemblywas placed into a battery case and the electrolyte prepared as above wasinjected into the case under reduced pressure, thus completing thefabrication of the prismatic rechargeable lithium battery cell.

For an electrolyte, 1.3M LiPF₆ dissolved in a mixed solvent of ethylenecarbonate, ethylmethyl carbonate, dimethyl carbonate, and fluorobenzenein a volume ratio of 30:55:5:10 was used.

COMPARATIVE EXAMPLE 2

A prismatic rechargeable lithium battery was prepared according to thesame method as in Comparative Example 1 except that 0.25 g of a compoundof the below Formula 17 for the additive was added to 5 g of a solutionincluding 1.3M LiPF₆ dissolved in a mixed solvent of ethylene carbonate,ethylmethyl carbonate, propylene carbonate and fluorobenzene in a volumeratio of 30:55:5:10 to prepare an electrolyte, and 2.3 g of theelectrolyte was added to the battery case.

EXAMPLE 1

A prismatic rechargeable lithium battery was prepared according to thesame method as in Comparative Example 1 except that 0.25 g of5-Fluoro-3H-benzofuran-2-one having the above Formula 6 for the additivewas added to 5 g of a solution including 1.3M LiPF₆ dissolved in a mixedsolvent of ethylene carbonate, ethylmethyl carbonate, propylenecarbonate and fluorobenzene in a volume ratio of 30:55:5:10 to preparean electrolyte, and 2.3 g of the electrolyte was added to the batterycase.

Lithium battery cells of Example 1 and Comparative Examples 1 and 2 wereovercharged at 2C to evaluate safety of the battery cells. Overchargewas performed by applying 2A of charge current to a fully dischargedcell for 2.5 hours. Measurement results of safety of the cells accordingto Example 1 and Comparative Examples 1 and 2 are shown in Table 1.

TABLE 1 Ignition Comparative Example 1 ◯ Comparative Example 2 ◯ Example1 X

As shown in Table 1, the battery cells according to Example 1 havesuperb safety characteristics, compared with the cells according toComparative Examples 1 and 2. The compound of the above Formula 17 has ahigh oxidation potential but induces ignition indicating bad safetycharacteristics.

A lithium battery including an electrolyte of the present inventionshows improved overcharge characteristics over the battery including aconventional non-aqueous electrolyte.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. An electrolyte for a lithium battery, comprising: a non-aqueousorganic solvent; a lithium salt; and an additive having the followingFormula 1:

wherein, each of X and Y is independently selected from the groupconsisting of O, S, CR₂, or NR wherein each R is independently selectedfrom the group consisting of H, halogens, and alkyl groups having acarbon number of less than or equal to 8, and neighboring alkyl groupsmay be combined to each other to form cyclic or heterocyclic structures,and X and Y are not both oxygen (O), each of R_(a) to R_(d) isindependently selected from the group consisting of H, halogens, alkoxygroups having a carbon number of less than or equal to 8, andunsaturated or saturated alkyl groups having a carbon number of lessthan or equal to 8, and neighboring alkyl groups may be combined to eachother to form cyclic or heterocyclic structures, and at least one ofR_(a) to R_(d) is a halogen.
 2. The electrolyte of claim 1, wherein R isselected from the group consisting of H, halogens, and C₁-C₈ alkylgroups.
 3. The electrolyte of claim 1, wherein the halogen is fluorine.4. The electrolyte of claim 1, wherein the additive is represented bythe following Formula 2:

wherein, each of R¹ and R² is independently selected from the groupconsisting of H, halogens, and alkyl groups having a carbon number ofless than or equal to 8, and neighboring alkyl groups may be combined toeach other to form cyclic or heterocyclic structures, each of R³ to R⁶is independently selected from the group consisting of H, halogens,alkoxy groups having a carbon number of less than or equal to 8, andunsaturated or saturated alkyl groups having a carbon number of lessthan or equal to 8, and neighboring alkyl groups may be combined to eachother to form cyclic or heterocyclic structures, and at least one of R³to R⁶ is a halogen.
 5. The electrolyte of claim 1, wherein the additiveis represented by the following Formula 3:

wherein, each of R_(a) to R_(d) is independently selected from the groupconsisting of H, halogens, alkoxy groups having a carbon number of lessthan or equal to 8, and unsaturated or saturated alkyl groups having acarbon number of less than or equal to 8, and neighboring alkyl groupsmay be combined to each other to form cyclic or heterocyclic structures,and at least one of R_(a) to R_(d) is a halogen.
 6. The electrolyte ofclaim 1, wherein the additive is represented by the following Formula 4:

wherein, each of R¹ to R⁴ is independently selected from the groupconsisting of H, halogens, and alkyl groups having a carbon number ofless than or equal to 8, and neighboring alkyl groups may be combined toeach other to form cyclic or heterocyclic structures, each of R⁵ to R⁸is independently selected from the group consisting of H, halogens,alkoxy groups having a carbon number of less than or equal to 8, andunsaturated or saturated alkyl groups having a carbon number of lessthan or equal to 8, and neighboring alkyl groups may be combined to eachother to form cyclic or heterocyclic structures, and at least one of R⁵to R⁸ is a halogen.
 7. The electrolyte of claim 1, wherein the additiveis represented by the following Formula 5:

wherein, each of R_(a) to R_(d) is independently selected from H,halogens, alkoxy groups having a carbon number of less than or equal to8, and unsaturated or saturated alkyl groups having a carbon number ofless than or equal to 8, and neighboring alkyl groups may be combined toeach other to form cyclic or heterocyclic structures, and at least oneof R_(a) to R_(d) is a halogen.
 8. The electrolyte of claim 1, whereinthe additive is selected from the group consisting of the compounds ofFormulas 6, 7, 9, 13, 14, 15 and combinations thereof:


9. The electrolyte of claim 1, wherein an amount of the additive is 0.01wt % to 50 wt % based on the total weight of electrolyte.
 10. Theelectrolyte of claim 1, wherein the lithium salt is selected from thegroup consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃,Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂)(where x and y are naturalnumbers), LiCl, LiI and combinations thereof.
 11. The electrolyte ofclaim 1, wherein a concentration of the lithium salt is 0.6 M to 2.0 M.12. The electrolyte of claim 1, wherein the non-aqueous organic solventis selected from the group consisting of carbonates, esters, ethers,ketones and combinations thereof.
 13. The electrolyte of claim 12,wherein the non-aqueous organic solvent is selected from the groupconsisting of dimethyl carbonate (DMC), diethyl carbonate (DEC),dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropylcarbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate (BC) and combinationsthereof.
 14. The electrolyte of claim 12, wherein the non-aqueousorganic solvent is a mixture of a cyclic carbonate and a linearcarbonate.
 15. The electrolyte of claim 1, wherein the non-aqueoussolvent is a mixture of a carbonate solvent and an aromatic hydrocarbonsolvent.
 16. The electrolyte of claim 15, wherein the aromatichydrocarbon solvent is represented by the following Formula 16:

wherein R¹¹ is a halogen or a C₁ to C₁₀ alkyl, and n is an integer from0 to
 6. 17. The electrolyte of claim 15, wherein the aromatichydrocarbon solvent comprises a solvent selected from the groupconsisting of benzene, fluorobenzene, toluene, trifluorotoluene,chlorobenzene, xylene and combinations thereof.
 18. The electrolyte ofclaim 15, wherein the carbonate solvent and the aromatic hydrocarbonsolvent are mixed in a volume ratio of 1:1 to 30:1.
 19. A lithiumbattery comprising: an electrolyte according to claim 1; a positiveelectrode including a positive active material being capable ofintercalating and deintercalating lithium ions; and a negative electrodeincluding an active material selected from the group consisting of amaterial being capable of intercalating/deintercalating lithium ions, alithium metal, a lithium-containing alloy, and a material being capableof forming a lithium-containing compound by reversibly reacting lithium.